RE: [PATCH 2/3] X86: Add a check to catch Xen emulation of Hyper-V

[KY Srinivasan <kys@microsoft.com>]

> -----Original Message-----
> From: Michael S. Tsirkin [mailto:mst@redhat.com]
> Sent: Wednesday, April 17, 2013 9:25 AM
> To: KY Srinivasan
> Cc: x86@kernel.org; gregkh@linuxfoundation.org; linux-kernel@vger.kernel.org;
> devel@linuxdriverproject.org; olaf@aepfle.de; apw@canonical.com;
> jasowang@redhat.com; tglx@linutronix.de; hpa@zytor.com;
> JBeulich@suse.com; bp@alien8.de
> Subject: Re: [PATCH 2/3] X86: Add a check to catch Xen emulation of Hyper-V
> 
> On Wed, Apr 17, 2013 at 02:12:12PM +0000, KY Srinivasan wrote:
> >
> >
> > > -----Original Message-----
> > > From: Michael S. Tsirkin [mailto:mst@redhat.com]
> > > Sent: Wednesday, April 17, 2013 8:49 AM
> > > To: KY Srinivasan
> > > Cc: x86@kernel.org; gregkh@linuxfoundation.org; linux-
> kernel@vger.kernel.org;
> > > devel@linuxdriverproject.org; olaf@aepfle.de; apw@canonical.com;
> > > jasowang@redhat.com; tglx@linutronix.de; hpa@zytor.com;
> > > JBeulich@suse.com; bp@alien8.de
> > > Subject: Re: [PATCH 2/3] X86: Add a check to catch Xen emulation of Hyper-V
> > >
> > > On Wed, Apr 17, 2013 at 01:20:58PM +0000, KY Srinivasan wrote:
> > > >
> > > >
> > > > > -----Original Message-----
> > > > > From: Michael S. Tsirkin [mailto:mst@redhat.com]
> > > > > Sent: Wednesday, April 17, 2013 3:06 AM
> > > > > To: KY Srinivasan
> > > > > Cc: x86@kernel.org; gregkh@linuxfoundation.org; linux-
> > > kernel@vger.kernel.org;
> > > > > devel@linuxdriverproject.org; olaf@aepfle.de; apw@canonical.com;
> > > > > jasowang@redhat.com; tglx@linutronix.de; hpa@zytor.com;
> > > > > JBeulich@suse.com; bp@alien8.de
> > > > > Subject: Re: [PATCH 2/3] X86: Add a check to catch Xen emulation of
> Hyper-V
> > > > >
> > > > > On Sun, Feb 03, 2013 at 05:22:38PM -0800, K. Y. Srinivasan wrote:
> > > > > > Xen emulates Hyper-V to host enlightened Windows. Looks like this
> > > > > > emulation may be turned on by default even for Linux guests. Check
> and
> > > > > > fail Hyper-V detection if we are on Xen.
> > > > > >
> > > > > > Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
> > > > >
> > > > > I'm very curious what's the specific bug that is fixed here?
> > > > > I went over the original discussion in
> > > > > https://patchwork.kernel.org/patch/2064331/
> > > > > and that's still not clear to me. Is there a configuration
> > > > > that is broken without this patch but starts working with
> > > > > this patch?
> > > > >
> > > > > It seems that one might want to use hyper-v emulation e.g. to test
> > > > > hyper-v code without using windows, so the functionality
> > > > > that this patch disables is not completely useless,
> > > > > so there should be a good reason for disabling it.
> > > > >
> > > > > Could you enlighten me please? :)
> > > >
> > > > Michael,
> > > >
> > > > If I recall correctly, the issue here was that Xen was enabling
> > > > Hyper-V emulation un-conditionally even for Linux guests.
> > >
> > > Okay so basically some Xen specific strangeness.
> > >
> > > > Clearly,
> > > > this was not the intention to begin with. This check was added to
> > > > ensure that while hosting Linux we would not un-intentionally enable
> > > > Hyper-V emulation in Xen.
> > >
> > > What if you want to intentionally enable Hyper-V emulation in Xen?
> > > There's no way to do this now, right?
> > >
> > > > Not all Hyper-V features are tagged with a
> > > > CPUID feature bit and so it was agreed that it would be better to
> > > > disable Hyper-V emulation when hosting Linux on Xen.
> > > >
> > > > Regards,
> > > >
> > > > K. Y
> > >
> > > I was just asking whether there are any specific examples of a bug
> > > or is this change done just in case?
> >
> > Ideally, if Hyper-V could be emulated in a plug compatible way, in that the guest
> would not know
> > the difference as to whether Hyper-V was being emulated or indeed it was
> Hyper-V underneath, I would agree with you.
> > Even in this case, I would argue that we would somehow give preference to the
> native implementation rather
> > than the emulated environment. Having done the initial emulation of Hyper-V
> on Xen, I know the current emulation of
> > Hyper-V is restricted to emulating a few features advertised via the CPUID bits.
> If I recall correctly, Xen did not implement
> > the enlightened TLB flush scheme. Furthermore, none of the Hyper-V
> emulations implement the I/O related infrastructure.
> > For Linux to run efficiently on Hyper-V, the I/O paths are significantly more
> important than what Hyper-V emulation there is
> > in Xen and I suspect in KVM. So, I don't think we want to run Linux on Xen/KVM
> that is emulating Hyper-V (cpuids).
> 
> No, it's a Xen specific bug.  KVM in a hyper-v mode still emulates PCI
> so can still use KVM PV devices which are all PCI devices. So there's no problem
> there.

Even Xen does not have a problem here; Xen would install its PV drivers in the guest.
So, the I/O infrastructure that would be installed in the guest would be based on the 
host.
> 
> Also, why override this in the guest? With time Xen hyper-v emulation
> might improve, meanwhile it's always possible to disable this in
> the hypervisor.
> 
> > The specific case I ran into that required this patch was when I moved the
> VMBUS interrupts to be delivered via a special
> > hypervisor vector (to support full interrupt distribution amongst all VCPUs in
> the guest). This feature is not tagged in the CPUID
> > feature bits and without this check, I would install an interrupt handler for the
> hypervisor vector that clearly did not make
> > any sense in anything other than Hyper-V. I suspect there will be more such
> situations as we go forward.
> >
> > Regards,
> >
> > K. Y
> 
> Why? Because there's no VMBUS on Xen? So check whether VMBUS exists?
On Linux, Hyper-V detection is used to load hyper-V related drivers (VMBUS etc.). For the case in
point, it was not clear to me how I could install the vmbus interrupt handler in a way that would
make sense when Xen was unconditionally emulating Hyper-V. 

Regards,

K. Y
> 
> > >
> > > > >
> > > > > > ---
> > > > > >  arch/x86/kernel/cpu/mshyperv.c |    7 +++++++
> > > > > >  1 files changed, 7 insertions(+), 0 deletions(-)
> > > > > >
> > > > > > diff --git a/arch/x86/kernel/cpu/mshyperv.c
> > > > > b/arch/x86/kernel/cpu/mshyperv.c
> > > > > > index 646d192..4dab317 100644
> > > > > > --- a/arch/x86/kernel/cpu/mshyperv.c
> > > > > > +++ b/arch/x86/kernel/cpu/mshyperv.c
> > > > > > @@ -30,6 +30,13 @@ static bool __init ms_hyperv_platform(void)
> > > > > >  	if (!boot_cpu_has(X86_FEATURE_HYPERVISOR))
> > > > > >  		return false;
> > > > > >
> > > > > > +	/*
> > > > > > +	 * Xen emulates Hyper-V to support enlightened Windows.
> > > > > > +	 * Check to see first if we are on a Xen Hypervisor.
> > > > > > +	 */
> > > > > > +	if (xen_cpuid_base())
> > > > > > +		return false;
> > > > > > +
> > > > > >  	cpuid(HYPERV_CPUID_VENDOR_AND_MAX_FUNCTIONS,
> > > > > >  	      &eax, &hyp_signature[0], &hyp_signature[1],
> &hyp_signature[2]);
> > > > > >
> > > > > > --
> > > > > > 1.7.4.1
> > > > > >
> > > > > > --
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